Carrier-signal receiver



April 21, 1942. +1. A. WHEELER ETAL CARRIERSIGNAL RECEIVER Filed Jan. 23, 1941 zebuhwn ATTORNEY Patented Apr. 21, 1942 v a UNITED-STATES PATENT OFFICE" W CARRIER-SIGNAL RECEIVER 111mm A. Wheeler, Great Neck, and Robert L. Freeman, Flushing, N. Y., assignors to Hazeltine Corporation, a corporation of Delaware Application January 23, 1941, Serial No. 375,5"74

7 Claims. (Cl; 250-) The present invention relates to carrier-signal receivers and,particularly, to an automatic frequency control system for such receivers.

While the invention. is of general application, it

is particularly adaptable toan ultra-high frequency carrier-signal receiver provided with an automatic frequency control system, the receiver having relatively little selectivity during the initial operation of tuning the receiver to a desired carrier signal but. greatly increased selectivity after the receiver. has-been accurately tuned to such carrier signal. r

Ultra-high-frequency carrier-signal receivers of the superheterodyne type are usually extremely selective and, therefore, difiicult to tune accurately to a desired carriersignal. Furthermore, after such a receiver has initially been carefully tuned, the accuracy of tuning is quite likely to be impaired soon afterward by the frequency drift inherent in the conventional forms of local heterodyneoscillator. :To facilitate accuracy in, the initial tuning operation and to maintaina high degree of accuracy of tuning once established, it is desirable that .the receiver be provided with. some form of. automatic frequency control system. The. use of an automatic frequency control systennwhich generally takesthe form. of an automatic. control of the heterodyne. oscillator frequency, is particularly advantageous where tuning ofthe receiver is. accomplished by mechanical or electrical tuning means initially preset to individual ones of provide a new and improved carrier-signal receiver having an automatic frequency control system which avoids one or more of the abovementioned disadvantages and limitations of the prior art systems. L

It is a further object of the invention to provide a carrier-signal receiver having' an automatic frequency control system and having a band-pass selector which initially has a1relatively wide pass band and in which the pass band is narrowed with the accurate tuning of the receiver to a desired signal.

In accordance with the invention, a carriersignal receiver comprises a carrier-signal translating channel including a band-pass selector of adjustable band width having a mean resonant which deviations may be substantial as coma numberof desired carrier signalsany one of M which may be selected at will as by the manual operation of a selected: tuning button.

Automatic frequency control system generally have a frequency detector which is responsive tothe frequencyof a carrier signal passed by one or more carrier-signal selectors. It is possible to make the pass band of the frequency detector quite broad and yet to retain the required degree of detector sensitivity. This broadness would be desirable to'facilitate the ease of the initialmanual tuning operation or to extend the permissible. limits of inaccuracy of tuning of the preset tuning means. not been possible" heretofore, however, since the effectiveness of the frequency detector over a band of frequenciesis limited by the pass band of the selector circuits which precede it and the latter necessarily is narrow-to ensure adequate selectivity of the receiver against undesired car- This has pared with the band width of the carrier signal. The selector initially has a pass band sufficiently wideto include the carrier signal and the aforesaid deviations. The receiver includes means responsive to the mean. frequencyof a carrier signal passed by the selector for reducing the deviations substantially to maintain the aforesaid normal predetermined relation and means for contracting'the pass band of the selector in response to the translation of a carrier signal thereby.

In accordance with a preferred form of the invention, a carrier-signal receiver of the super heterodyne type comprises means for converting a received carrier signal to an intermediatefrequency carrier signal normally having a predetermined mean frequency but subject to undesired deviations therefrom which may be substantial as compared with the band widthof the carrier signal. The receiver is provided with an intermediate-frequency carrier-signal translatingchannel including a band-pass selector of adjustable band width having an initial pass band sufficiently wide to include the carrier signal and the aforesaid undesired deviations. The receiver also includes means responsive to the mean frequency of an intermediate-frequency carrier signal passed by the selector for controlling the converting means to reduce the aforesaid undesired deviations and means for contracting the pass band of the selector in response to the translation of an intermediatefrequency carrier signal thereby.

For a better understanding of the present invention, together with other and further objects thereof, reference is had to the following description taken in connection with the accompanying drawing and its scope will be pointed out in the appended claims.

In the accompanying drawing, the single figure is a circuit diagram, partly schematic, of a complete carrier-signal receiver of the superheterodyne type embodying the present invention.

Referring now more particularly to the drawing, there is represented a complete carrier-signal receiver of the superheterodyne type embodying the present invention in a preferred form. In general, the receiver includes a radiofrequency selector and amplifier I0 having its input circuit connected to an antenna system I I, I2, and having its output circuit connected to an oscillator-modulator I3. Connected in cascade With the oscillator-modulator I3, in the order named, are an intermediate-frequency selector and amplifier I4 of one or more stages, a detector and A. V. C. supply I5, an audio-frequency amplifier I6of one or more stages, and a sound reproducer I'I. Also connected to the output of the intermediate-frequency amplifier I4 is a frequency detector I8, the output of which is, in turn, coupled to the input circuit of a frequency shifter 28. The latter includes a frequency-control circuit coupled with the oscillator-modulator The receiver is provided with electrical or me chanical preset tuning means indicated schematically at 29, comprising a plurality of tuning buttons by which the selector of unit ID may be tuned to any desired one of a number of preselected carrier signals.

An automatic amplification control or A. V. C. circuit is connected between the output circuit of the A. V. C. supply of unit I5 and the input circuits of one or more of the tubes of the radiofrequency amplifier I 0, the oscillator-.modulator I3, and the intermediate-frequency amplifier I4, in conventional manner.

It will be understood that the various units just described may, with one or two exceptions presently to be considered, be of a conventional construction and operation, the details of which are well known in the art, rendering detailed description thereof unnecessary. Considering briefly the operation of the receiver as a whole, a desired carrier signal is selected and amplified by the radio-frequency amplifier I0, converted to an intermediate-frequency carrier signal in the oscillator-modulator I3, amplified in the intermediate-frequency amplifier I4, and detected by the detector I5, thereby to derive the audio-frequency modulation components. The audiofrequency components are, in turn, amplified in the audio-frequency amplifier I5 and are reproduced by the sound reproducer I! in a conventional manner.

From the intermediate-frequency carrier signals applied to the input of th frequency detector I8, there is derived in the output circuit of the frequency detector a control potential which is applied to the frequency shifter 28 to control the frequency of oscillations generated by the oscillator of unit I3, thereby to maintain the frequency of the intermediate-frequency carrier signal substantially constant at a predetermined value in a conventional manner.

The automatic amplification control or A. V. C. bias derived from the A. V. C. supply of unit I5 is effective to control the amplification of one or more of the units l0, I3, and I4 to maintain the signal input to the detector of unit I5 within a relatively narrow range for a wide range of received signal intensities.

Referring now more particularly to the portions of the system embodying the present invention, the selector circuit of radio-frequency amplifier I8 has a wide band-pass characteristic, as indicated by curve 38, which is wider than desirable for freedom from interference from undesired carrier signals adjacent to the desired carrier signal and from excessive noise components. The intermediate-frequency amplifier I4 comprises a carrier-signal translating channel including a band-pass selector of adjustable band "width having a mean resonant frequency normally bearing a predetermined relation to the mean frequency of the intermediate-frequency carrier signal to be translated thereby. The relation between these mean frequencies is subject to deviations from the aforesaid predetermined relation which may be substantial as compared with the band width of the carrier signal. The band-pass selector of amplifier I4 may be of any suitable type wherein the pass band thereof is contracted in accordance with the value of a control bias applied thereto, for example, of the type disclosed in United States Letters Patent No. 2,152,618 issued to Harold A. Wheeler on March 28, 1939. The band-pass selector of that patent includes input and output resonant circuits coupled by a forward coupling tube and a backward coupling tube, the contraction of the pass band of the selector being accomplished by applying to at least one of the tubes 2. negative control bias to reduce the transconductance thereof with increasing amplitude of signal output and control bias. The band-pass selector of the present invention is designed to have initially a pass band sufiiciently wide to include both the carrier signal to be translated by the selector and also the deviations to which reference has previously been made, as indicated by curve 32.

The A. V. C. bias derived from unit I5 varies with the mean amplitude of the carrier-signal output from unit I4 and is applied to one or more of the tubes of the band-pass selector system of the amplifier I4 in such sense as to adjust the pass band of the selector inversely in accordance with the mean amplitude of the carrier signal. If the circuit of the above-mentioned Patent 2,152,618 is employed, it is preferable that this bias be applied to the backward tube or tubes, to secure the most complete reduction of the pass band on moderately-weak signals. A signal of sufiicient strength operates to reduce the pass band of the selector substantially to the minimum permissible value for reducing interfering disturbances. The contracted pass band of the selector is much less than the initial pass band and is preferably only sufiiciently wide to pass the modulation band of the desired carrier signal, the reduced pass band being indicated by the broken-line curve 32a of the drawing.

The constants of the control-bias circuit of the band-pass selector, to which the control bias from unit I5 is applied, may be proportioned to have a time constant-such that the contraction of the pass band of the selector is relatively slow as compared with the frequency compensation effected by unit 28.

The pass band of the frequency detector I8 is preferably at least as wide as the initial pass band of the band-pass selector of the interme- 2,280,139, diate-frequency amplifier M, .as indicated by the .curve 33 which is the"frequencyeresponse curve of the frequency detector;

In considering the operation of the invention, itwill be assumed that the receiver is not tuned to ,anyf'carrier signal and it is desired manthe received carrier signal or frequency drift of the' 'local oscillator is immediately corrected by the action of the automatic frequency, con-,

trol system.

uallytotune'the receiver to aparticular carrier signal; The receiveris tuned .into such proximity with the desired carrier signal that the latter is within the Wide band-pass character istics of the radio-frequency amplifier Ill, whereupon a I corresponding intermediate-free quency carrier signal appears within the wide band-pass characteristic 32 of the band-pass selector of intermediate frequency amplifier H.

A bias-control voltageis thereupon derived in theoutputof theifrequencyydetector I8 and is applied "to the frequency shifter 28 which operates ,soiflto change the frequency of oscillations generated by the oscillator offunit 13 that the frequency of the intermediate-frequency carrier signal is adjusted to a value approximately equal to the meanresonant frequency of the band-pass selector of amplifier M. The time constantof this automatic frequency control is preferably J very short, unless the carrier signal is of the frequency-modulation type,in which case this time constant is greater than the periods of the low frequencies of modulation. Under .the assumed conditions, a controlbias is also developed by the A. ,V. C. supply of unit 15, but the action of this bias in controlling the band-pass selector of amplifier I4 to contract the pass band is delayed by the relatively long time constant ofithis circuit so that the pass band of thecselector does not become fully conv tracted until after the automatic frequency con.-

trol systemcomprising detector l8 and the frequency shifter 28 has shifted .the frequency of the intermediate-frequency"carrier signal substantially to the mean resonant frequency of the band-pass selector of unit I4. It is desirable that the contraction, of the pass band of the selector be not only delayed withlrespect to the frequency. correction of the intermediate-fre- .4 quency carrier signal, but also that the pass band contraction occur substantially instantaneously; i

Since ,distortion of the desired carrier signal or. interference therewith .may be caused by undesired frequency deviations between the mean resonant frequency of. s the intermediatefrequency selector of unit l4 and the mean frequency of the intermediate-frequency carrier signal, it may, in general, be said that the receiver asa whole isaccurately tunedtothe desired carrier signal when such deviations are reduced to a minimum eventhough the units [0 and I3 may be somewhat detuned from the received carrier signal. Whenlthis condition is established, orshortly subsequent thereto, the" pass band of the selector of amplifier l4 auto matically becomes fully contracted, whereupon the receiverfloperates with a high selectivity against interference ibyundesired' carrier sig nals adjacent to that of the desired carrier sigl nal and 1 against excessive noise components.

It may be noted that, once the automatic frequency control system comprising detector I8 and frequency shifter 28 has accurately tuned the receiver to a desired carrier signal, the accuracy of such tuning is thereafter maintained, even though the receiver is highly selective by virtue .of thecontractedpass band of the bandpass selector, since any change of frequency of ficult to maintain such accuracy of tuning over rier signal, the A. V. C. (control bias from unit [5 disappears, the pass band of the bandpass selector of unit l4 thereupon expands to its initial Width, and the receiver thereafter has an over-all wide pass band characteristic and is conditioned to be tuned with facility to any desired carrier signal. a

The frequency detector land the frequency shifter 28 thusfcomprise meansuresponsive to the mean frequency of, a carrier; signal passed by the band-pass selector of unit for reducing the deviations from apredetermined desired relation between the mean frequency of the. in-

termediate-frequency carrier, signal and the mean resonant frequency of the selector of unit I 4, thereby substantiallyto maintain the de-:

sired normal predetermined relation therebetween. That is, it is desired that the 'intermediate-frequency carrier signal have a mean frequency substantially equal to the mean a reso nant frequency of the band-pass selector of unit} l4 and that the automatic frequency control a selected push-button" of the preset-"tuning v means 29. A carrier-signal receiver embodying the invention has the advantage that the most selective part of the receiver automatically is accurately tuned to a desired carrier signal, even though the less selective radio frequency amplifier i0 and oscillator-modulator i3 are tuned only approximately to the desired carrier ;signal by the operation. of the preset tuning means, 29. i This is especially important in the higher frequency bands, where it is relatively difiicult initially topreset the tuning means '29 withsufficient precision to selected carrier si g nalsiand where, even though the tuning means 29 has initially been accurately preset; it is difany appreciable period of time.

A carrier-signal receiver embodying the invention has the additional advantage that the initial over-all broad pass-band characteristic of the receiver greatly facilitates the ease by which the receiver may be tuned to a desired carrier signal.

That is, the initial broad pass-band characteristic of the receiver permits the automatic frequency control system to operate over a pass band materially wider than that heretofore possible where the receiver has necessarily had a relatively narrow pass-band characteristicto avoid undue interference from undesired carrier signals in proximityto a desired received carrier signal. While the above description of the invention and its operation has, in general, been treated as though the invention were embodied in an amplitude-modulated carrier-signal receiver, the invention is equally suitable for use in a frequency-modhlated carrier-signal receiver or in a car'- rier-signal receiver adapted to receive a desired carrier signal and a single modulation sideband thereof,- Y

Ondetuning the receiver, from a'received car Wherev the carrier-signal receiver' is of the fre quency-modulated: type-, the unit I comprises a frequency-modulation detector and may include; an amplitude-limiting system of conventional form. Alternately, in' this form of. receiver either one of thezunits =l5' or lfl-maybe-omittedand the input of the audio-frequency amplifier lB-is then connectedto theoutput of the unit retained. In .the latter event, the --A. V. :0. supply is included in the retainedunit'andis arranged to be 10 responsive tothe amplitudeof the intermediate- I frequency carrier signal in conventional-manner.

In the event thatthe carrier-signal receiver is of the single sideband type, that is, adaptedtoreceive a :carrier signal anda single modulation sideband thereof ,the intermediate-frequency car-. rier signal has a frequency near'one edge of the passebandcharacteristic 32a of 1 the selector of unit;l4.and the automatic frequency control system', comprisingthe frequency detector 18 and the frequencyshifter-28, 150 controls the-operation.of the oscillator-modulatorl3 thatthe frequency of the intermediate-frequency carrier. sig nal is adjusted to. and: maintained; atthis par- I ticular relationship with respect to the pass-band characteristic'of .the selector of unit I4.

While there has. been described what is at present considered to be the preferred embodiment of this invention, it will be obvious to those skilled in the art that various changes and modifications may be .made therein without depart-' ing from the invention, and it is,.therefore, aimed in the appended claims to cover all such changes and modifications as fall. within the-true spirit and scope of the invention.

What is claimed is:

l. A carrier-signal receiver comprising,= a carrier-signal translating channel including a bandpass selector of adjustable band width having a mean resonant frequency normally bearing a pre- .10 determined relation to the mean frequency of a carrier signal to be'translated thereby, the relation between-said mean frequencies being subject to deviations from said predetermined rela'-' tion which may be: substantial as compared with the band width of .said signal, said selector initially having a pass. band sufficiently wide to include said signal and saiddeviations, means responsive to the'mean'frequency of a carriersignal passed by said selector for reducing said deviations substantially to maintainisaid normal predetermined relatiornand means for contracting the pass bandof said .selector in response to the translation of a carrier. signal thereby.

2. A carrier-signal receiver comprising, a carrier-signal translating channel including a bandpass selector of adjustable band width hav-ing a mean resonant frequency normallyequal to the mean frequencyof ,a carrier signal to be translatedthereby, the difference between said .mean frequencies being subject to deviationsfromzero which may be substantial-as compared with the band width-of saidsignal, said selector :initially having a pass bandsufiicientlywide to include W said signal and said deviations, means responsive. to the mean frequency of a, carrier signalpassed by. said selector for reducing saiddeviations,.and means ,for contracting the pass band of said selectorin response to the translation of a carrier signal thereby.

3. Acarrier-signal receiver comprising-a car-' rier-signal translating channel including a band-" pass selector of adjustable bandwidth having a mean resonant frequencynormally bearing a press determined relation to the mean frequencytofra 7 carrier signal to be translated thereby, the rela tionv between .said mean frequencies being sub.- ject to deviations from said predetermined relation which. may be substantial as compared with the .band: width of. saidcarrier signal, said selector initially having a pass band sufliciently wide to include said signal and said deviations, means responsive to the meanfrequencyof a carrier signalpassed by said selector for reducing said deviations substantially to maintain said normal predetermined. relation, and means having a time constant at least as long as that of said reducingmea-ns for contracting the pass band of said selector in response to the translation of a carriersignal thereby.

4. A carrier-signal receiver comprising, a carrier-signal translating channel including a bandpass selector of adjustable band width having a mean resonant frequency normally bearing a predetermined relation to the mean frequency of a carrier signal to be translated thereby, the relation between said mean frequencies being subject to deviations from said predetermined relation which may be substantial as compared with :rthe band width of saidv carrier signal, said selector initially having a pass band sufi'iciently wide to include said signal and said deviations:

means having a relatively short time constant responsive to the mean frequency ofv a carrier 5: signal passed by said selector for reducing said deviations.substantially, to maintain said normal predetermined relation, and means having a relatively long time constant for contracting the pass band of said selector in response to the transla- ;;:.tion of a carrier signal thereby.

5. A carrier-signal receiver comprising, a carrier-signal translating channel including a bandpass selector of adjustable band width having a mean resonant frequency normally bearing a presdetermined relation to the mean frequency of a carrier signal to be translated thereby, the relation between said mean frequencies being subject to deviations from said predetermined relation which may be substantial as compared with ,the band width of said carrier signal, said selector initially having a pass band sufficiently wide to include said signal and said deviations, means responsive to the mean frequency. 'of a carrier signal passed by said selector for reducing said deviations substantially to maintain said normal predetermined relation, and means responsive to the translation of a carrier signal by said selector for contracting the band width of said selector to that. corresponding to the band width of said carriertsignaL.

6. A carrier-signalreceiver comprising, a carrier-signal translating channel including a bandpass selector of adjustable band width having a mean resonant frequency: normally bearing a predetermined relation'tothe mean frequency of a carrier signal to be translated thereby, the relation between said mean frequencies being subject to deviations from said predetermined relationwhich may be substantial as compared with the band width of said carrier signal, said selector initially havinga pass band sufficiently wide to include said signaland said deviations, means responsive to the mean frequency of a carrier signal passed by said selector for reducing said deviations substantially to maintain said normal predetermined relation, and means responsive to the:translati0n;of a carrier signal by said selector for'adjusting-the band-pass of said selector: inversely in. accordance with the mean amplitude' of' said carrier signal;

7. A superheterodyne carrier-signal receiver comprising, means for converting a received carrier signal to an intermediate-frequency carrier signal normally having a predetermined mean frequency but subject to undesired deviations therefrom which may be substantial as compared with he band width of said carrier signal, an inter ediate-frequency carrier signal translating annel including a band-pass selector of adjust ble band width having an initial pass band 20 sufllciently wide to' include said intermediatefrequency carrier. signal and said undesired deviations, means responsive to the mean frequency of an intermediate-frequency carrier signal 

